From 84ee3b1cd87b3a9161eee3634c529eb835703fba Mon Sep 17 00:00:00 2001 From: "Paul R. Eggert" Date: Mon, 26 Jul 2010 10:56:32 -0700 Subject: [PATCH] timespec: use cast and not conditional, as truncation isn't possible * lib/timespec.h (timespec_cmp): Use cast to pacify gcc -Wconversion instead of a conditional. Comment about the situation in more detail. This undoes most of the 2009-10-29 patch. --- ChangeLog | 7 +++++++ lib/timespec.h | 32 ++++++++++++++++++++++++++++---- 2 files changed, 35 insertions(+), 4 deletions(-) diff --git a/ChangeLog b/ChangeLog index 8fb3e771f..897387c6a 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,10 @@ +2010-07-26 Paul R. Eggert + + timespec: use cast and not conditional, as truncation isn't possible + * lib/timespec.h (timespec_cmp): Use cast to pacify gcc -Wconversion + instead of a conditional. Comment about the situation in more detail. + This undoes most of the 2009-10-29 patch. + 2010-07-18 Bruno Haible unistr/u8-strchr: Optimize non-ASCII argument case. diff --git a/lib/timespec.h b/lib/timespec.h index 81b342300..175de7f26 100644 --- a/lib/timespec.h +++ b/lib/timespec.h @@ -22,15 +22,39 @@ # include /* Return negative, zero, positive if A < B, A == B, A > B, respectively. - Assume the nanosecond components are in range, or close to it. */ + + For each time stamp T, this code assumes that either: + + * T.tv_nsec is in the range 0..999999999; or + * T.tv_sec corresponds to a valid leap second on a host that supports + leap seconds, and T.tv_nsec is in the range 1000000000..1999999999; or + * T.tv_sec is the minimum time_t value and T.tv_nsec is -1; or + T.tv_sec is the maximum time_t value and T.tv_nsec is 2000000000. + This allows for special struct timespec values that are less or + greater than all possible valid time stamps. + + In all these cases, it is safe to subtract two tv_nsec values and + convert the result to integer without worrying about overflow on + any platform of interest to the GNU project, since all such + platforms have 32-bit int or wider. + + Replacing "(int) (a.tv_nsec - b.tv_nsec)" with something like + "a.tv_nsec < b.tv_nsec ? -1 : a.tv_nsec > b.tv_nsec" would cause + this function to work in some cases where the above assumption is + violated, but not in all cases (e.g., a.tv_sec==1, a.tv_nsec==-2, + b.tv_sec==0, b.tv_nsec==999999999) and is arguably not worth the + extra instructions. Using a subtraction has the advantage of + detecting some invalid cases on platforms that detect integer + overflow. + + The (int) cast avoids a gcc -Wconversion warning. */ + static inline int timespec_cmp (struct timespec a, struct timespec b) { return (a.tv_sec < b.tv_sec ? -1 : a.tv_sec > b.tv_sec ? 1 - : a.tv_nsec < b.tv_nsec ? -1 - : a.tv_nsec > b.tv_nsec ? 1 - : 0); + : (int) (a.tv_nsec - b.tv_nsec)); } void gettime (struct timespec *); -- 2.11.0